CSS is complicated but it's not as complex. It's just a matter of throwing enough sufficiently competent developers at the problem.
Ladybird doesn't even have a JIT right now. They used to, but it got taken out because as best as I can tell, nobody on the project knows how to write one.
And if you think that writing a JS interpreter is the only hard part of a browser engine, have I got news for you.
Why should anyone work for free? Because they love the Web and hate Google's stranglehold over it.
Why would they work for free even given this? It's not something you hack on in an evening or two
It may be possible for a scripting language with at least the same features as LuaJIT to be as fast as LuaJIT in less than 85K lines of code.